More than 300 shells fired by rebels hit western, government-held neighborhoods in Aleppo.

Rebels launched an attack against government positions in the historic center of Aleppo on Monday in response to an offensive that cut a road leading into the opposition-held sector of the city, monitors and insurgents said.

The attacks, which involved the shelling of government-held neighborhoods and intense street fighting, came just days after the advance by the government side towards the Castello Road. Rebels were relying on the road for supplies and access, and its severing effectively put opposition-held areas under siege, where at least 250,000 people live.

The city, Syria’s largest before the war, has been a main battleground in the country’s five-year civil war, and has seen an escalation of fighting after peace talks and a fragile ceasefire broke down earlier this year. Its capture would be a strategic prize for the Damascus government, which controls the major population centers in western Syria, apart from rebel-held areas of Aleppo, and the city of Idlib. Rebels also control pockets of territory elsewhere in western Syria.

Kurdish forces control vast areas along the nearby Turkish border, and ISIS holds territory in the east. Early on Monday more than 300 shells fired by rebels hit western, government-held neighborhoods in Aleppo killing five people and wounding dozens more, the Syrian Observatory for Human Rights said. State television said eight people had been killed and that the bombardments had brought down buildings.

The assault was “a response to the (government) attempts to advance,” Zakaria Malahifji of the Aleppo-based Fastaqim group told Reuters. He said insurgent fighters had already made gains, and that much of the fighting was taking place in Aleppo’s historic Old City. A witness said there were fierce clashes at close range near the ancient citadel.

A Syrian state TV correspondent said there had been heavy fighting since the morning, and that the army had seen off rebel attacks, killing many fighters. Syrian warplanes bombarded rebel-held areas in the city, the British-based Observatory said. A pro-Damascus TV channel said Russian warplanes, backing the government, were bombing areas north of the city, near the Castello Road.

A statement from the political office of the Jabha Shamiya rebel group dated Sunday said the government advance near the road came with the support of government “allies of various nationalities, with Russian air cover and with firepower of an unprecedented intensity.”

Assad is supported by Moscow, which launched an air campaign in Syria in September, Iranian troops and Lebanese Shi’ite Hezbollah fighters. Hezbollah has said it sees Aleppo as the most important battle in Syria, equating it to the defense of Damascus itself, and hundreds of its fighters have died in the conflict. Another mid-ranking commander, Samir Awada, was killed in recent days, a security source told Reuters.

Assad’s allies say they are battling the al Qaeda-linked Nusra Front in Aleppo. But groups fighting under the banner of the Western-backed Free Syrian Army (FSA) say they control the rebel-held part of the city. Nusra Front said in a statement it had launched an attack in central Aleppo, and had made advances towards a market in a government-held area.

Malahifji said there was only a small Nusra Front presence in Aleppo. The latest violence erupted during a 72-hour nationwide ceasefire announced by the Syrian army. The Observatory – a British-based organization which says it gets information from a network of sources on the ground – said a previous ceasefire had been a ruse and had been exploited to stage the Castello Road attack.